This invention relates to an improvement in particle beam lithography systems used for processing a workpiece, such as a semiconductor wafer or mask, and particularly, to an improvement in the means for calibrating such systems.
Heretofore, in the prior art particle beam lithography systems, a reference and calibration grid of copper, connected to the X-Y stage, was used as a fiducial for calibrating such systems. The grid has essentially three functions: (a) to serve as a global reference location which is periodically used for the detection and quantification of stage drift over time, (b) as a reference for writing plane location, providing the particle beam column with a standard to focus upon, and (c) to serve as a reference to set up the particle beam column--to determine beam shape, beam current, beam uniformity, and beam rotation. The prior art copper grid was originally designed for use in a scanning electron microscope and then later used in particle beam lightography systems simply because it was convenient but the copper grid has a number of deficiences. The grid itself is fragile and quite small, and therefore difficult to handle, and the square holes have a period (center-to-center) of 63.5 microns which required extra calibration to be coordinated in a particle beam system having binary electronics. In addition, the corner radii of the square holes are typically several microns, on the order of the distance from the corner where the X-Y scans are made, and the edge roughness of the copper grids is variable and typically larger than a tenth of a micron. The orthogonality of adjacent grid bar edges in the copper grids is also variable and larger than the requirements of the lithography system. Finally, because of the thinness of the copper grids, these were difficult to mount with uniform flatness.
It is therefore a primary object of this invention to improve particle beam lithography systems by incorporating a reference and calibration grid into such systems which overcomes the deficiencies of the prior art calibration grids.
As used herein, the "grid" is that part of a larger structure, herein called a "silicon die", used for reference and calibration of the lithography system.